The present invention relates to a device for shearing gobs from one or a plurality of strands of material in plastic condition, for instance molten glass, in which the shearing of each gob is accomplished by two cooperating shear blades movable toward and away from each other.
In a known device of this kind (DT-OS 2,304,009) each cutting blade consists of a relatively thin light and resilient blade body made from high speed steel in which the total surface of the blade with the exception of the ground cutting edge and a zone bordering thereon is covered with a layer of material having a high heat conductivity. This layer of material is relatively thin and consists for instance of an electrolytically applied silver layer. In addition, a cooling tube of copper is connected at least at one side of the shear blade with the aforementioned material layer in a heat conductive manner. Cooling water is circulated through this cooling tube.
The disadvantages of this construction are that the heat from the glass strand is transferred into the cutting edge and the bordering zone of the high speed steel, which has a relatively small heat conductivity and likewise a very small heat conducting cross-section. The very thin cutting edge is, due to its geometric form, subjected to the full heat load. Thereby, the combination of low heat storing capacity due to the small mass of the blade at the zone bordering the cutting edge, and the small heat conductivity of the high speed steel is especially disadvantageous.
Furthermore, the cutting edge and a relatively large zone of the blade bordering the cutting edge come in contact and therewith in heat exchange with the hot glass strand. An undue high temperature at the cutting edge and therefore a limitation of use of the known device will result at high cutting frequencies since the better heat conducting material layers, which are spaced from the cutting edge, can also, due to their small thickness, only absorb relatively small amounts of heat per time unit to conduct such heat to the water cooling tubes, which are necessary in the known device. Therefore, high temperature gradients will necessarily form in the shear blade. Since, on the other hand, for technological reasons a maximum temperature of the cutting edge of about 620.degree. C. is not to be surpassed in order to avoid sticking of glass to the cutting edge, the use of the known device, despite the necessary cooling tubes to be connected thereto, has only limited application. With the known device it is especially not possible to produce relatively heavy gobs with sufficient cutting frequencies, which gobs are often required in the industry for making hollow glass articles.